Optical radiation devices

ABSTRACT

A device, particularly an antenna device, for emitting a beam of optical radiation to irradiate a remote target area includes an optical source having a limited wavelength band, and a hologram pattern positioned in the path of a light beam originating from the source, the pattern being selected to produce a composite beam having a predetermined shape or far field pattern conforming to the target area and/or a predetermined distribution of light intensity in the target area. Where the optical source is a point source, the hologram pattern selectively retards the phase of respective components of the incident wavefront to produce a composite beam having a far field pattern that precludes focusing of the beam into a single small spot. The device has particular application in a telecommunication system wherein a data signal is transmitted through free space by an optical beam, at least one characteristic of the beam being controlled by the hologram pattern in the path of the beam.

This application is a 371 of PCT/GB93/01591 filed on Jul. 28, 1993.

This application is a 371 of PCT/GB93/01591 filed on Jul. 28, 1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a device, particularly an antenna device, foremitting a beam of optical radiation to irradiate a remote target area.The invention also concerns an optical free space communication systemwhere an antenna device is transmitting a signal over distances rangingfrom around half a meter up to several kilometers and where the remotetarget area is at least 0.5 m².

2. Related Art

It is well known that a laser can be used to produce a sharply definedand intense beam of infra-red radiation, and that a conventional lenscan spread this beam out over a target area. Other point sources, suchas infra-red light emitting diodes, can also produce sharply defineddirectional beams and are used, for example, to illuminate target areasin remote television and video control apparatus.

In general, however, when irradiating a remote target area with opticalradiation, much of the radiation from the original source inevitablyfalls outside the target area and is wasted. Moreover, although theangle of divergence of a beam can be controlled, there is little or nocontrol over the intensity distribution within the beam or he shape ofthe beam envelope. It is particularly difficult to illuminate a square,rectangular or other non-circular target area with a uniform intensitybeam. In an optical free space communication system where the radiationis carrying a telecommunication signal, the spillage of energy can alsoresult in inadvertent detection of the signal by a detector outside thetarget area.

With intensely bright optical sources, such as lasers, there is also arisk that the infra-red radiation could be inadvertently focused by alens, for example a binocular lens, on to the skin, or worse still, theretina, and cause permanent damage. This latter problem is particularlyacute at high power levels i.e. levels exceeding a few milliwatts andrestricts the possible use of high power sources, including both lightemitting diode (LED) and laser emitters, in an optical free spacecommunication system, or in any application where optical radiation isemitted into a populated area.

A paper entitled "Transforming a circular laser beam into a square ortrapezoid-almost" (Optical Engineering Vol. 31, No. 2, February 1992 pp.245-250) discusses how a computer-generated hologram positioned in thepath of a laser beam can be arranged to transform the shape of the beamfrom, say, a round beam into a square beam with rounded corners. Thetransformation is based on the phenomenon that an aberrated laser beamwill change shape as it propagates.

A further pacer entitled "Efficient optical elements to generateintensity weighted spot arrays: design and fabrication" (Applied OpticsVol. 30, No. 19, pp. 2685-2691), discusses the design and fabrication ofholographic beam splitters for producing multiple beams from a singlecoherent beam. The computer-generated patterns are made into surfacerelief diffraction elements or phase gratings by electron-beamlithography followed by plasma etching into quartz glass. Such elementsare therefore expensive to produce and are for use in laboratory and inoptical parallel computing systems where the multiple beams provide theoptical power supply to arrays of modulators or logic devices.

SUMMARY OF THE INVENTION

According to the present invention such elements are designed so thatwhen the beam splitter hologram pattern is positioned in the path of abeam originating from a coherent optical source emitting radiationpredominately in a wavelength band having an upper limit less than twicethe lower limit, the pattern scatters the incident wavefront into amultiplicity of beams at different angles and out of phase with oneanother such that the beams cannot be refocused by a lens to reproducean image of the source.

In addition, when the element is used in an optical free spacecommunication system, the optical radiation is modulated with atelecommunication signal and the hologram pattern is designed to producea composite beam having a particular shape conforming to a remote targetarea at least 0.5 meters from the halogram pattern.

Accordingly, the device is safe to use, even at high power levels,and/or it has the ability to direct light accurately into a remotetarget area of a particular shape while also controlling (if required)the intensity distribution across the area. In an optical free spacecommunication system, the distance of the target area from the opticalsource can range from 0.5 meters up to several kilometers.

The potential applications of the device are therefore considerable.

Moreover, we have found that an original surface relief hologram patternetched for example, in quartz glass, can be satisfactorily replicated ina low-cost substrate using established mass-production embossingtechniques. It therefore becomes possible to build up large surface areahologram patterns by stepping out the master pattern over the low-costsubstrate, or by combining a plurality of the replica patterns obtainedfrom one master. This means that the techniques and devices so farconfined to the laboratory and specialised computer applications wherethe computer-generated holograms generally have surface areas of around2 cm² can now be used on a much larger scale, with holograms typicallyhaving surface areas of around 100 cm², in a wide range of consumerproducts and telecommunication systems.

One such application would be in an optical free space communicationsystem since the device allows safe transmission of greatly increasedamounts of power while retaining the advantages of a laser or LEDsource. In this case, the device could act as an antenna either inside aroom or outside at least one building and the target area might then bea particular sector of a room or at least a portion of the building orbuildings. For example, it might be a single floor in an officebuilding, an individual window or windows in a building or block ofbuildings, or a single building or row or block of buildings in astreet. Advantageously, the antenna could be positioned at the top of apole, or a series of poles, in a street as in the present telegraph polesystem.

Other possible applications would be in television/video remotecontrollers where the light should at least point in the generaldirection of the detector on the TV or video recorder, car brake lightswhere the light should point in a generally backwards direction and inmuseums or art galleries where a signal carrying information relating toa particular picture or museum item could be directed by an infra-redbeam into an area immediately in front of the picture or item fordetection by the wearer of a personal headset.

If the hologram pattern is being used primarily to inhibit focusing ofthe light, the far field width of the light, or the spread of the beammay not necessarily be any greater than without the hologram. In thiscase, the only effect of the hologram is to make the light impossible tofocus, without necessarily changing the shape or direction of the beam.The hologram is effectively acting as a diffuser. The size of theindividual cells in a repeating pattern of cells forming the patternmight then be larger than when the hologram is being used to direct thebeam into a particular defined target area.

To accommodate the wavefront of a diverging beam, the hologram patternis conveniently repeated to form a periodic repeat pattern of a singlecell, each cell producing an array of beams. The repeating patternbreaks up the distribution of intensity in the target area into apattern of individual spots, but these spots can merge into one anotherif the incident beam is diverging. In this case the dimensions of eachcell control the angular spread of the array of beams, and the smallerthe cell the greater is the angular spread. The angular spread is equalto the wavelength of the light divided by the size of the cell oropening through which the light passes.

By a limited wavelength band we mean a band having upper and lowerlimits, particularly, where the wavelength of the upper limit is lessthan twice that of the lower limit. In addition, the term "optical" isintended to refer not only to that part of the electromagnetic spectrumwhich is generally known as the visible region but also the infra-redand ultraviolet regions at each end of the visible region.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, some embodiments of the invention areillustrated in the accompanying drawings in which:

FIGS. 1 and 1a are diagrammatic sketches of an optical radiation deviceembodying the invention;

FIGS. 2a-2c illustrate three possible hologram phase patterns for use inthe device of FIG. 1 to produce three differently shaped far fieldpatterns;

FIG. 3 illustrates diagrammatically an arrangement for illuminating atarget area consisting of four juxtaposed square cells using four of theradiation devices in FIG. 1 grouped together;

FIG. 4 illustrates diagrammatically an arrangement using the device ofFIG. 1 as an antenna device for illuminating a single building in a rowof buildings;

FIG. 5 is similar to FIG. 4 and shows an arrangement for illuminating arow of buildings in a street;

FIG. 6 shows an arrangement for illuminating selected houses in astreet; and

FIG. 7 shows an arrangement having multiple sources at differentwavelengths for respectively illuminating three adjacent cells in thetarget area.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring first to FIG. 1, the radiation emitter includes a laser diodesource 10 enclosed in a housing 11. The front of the housing is open ortransparent such that light from the source 10 is incident on a hologram12 positioned over the front. The housing 11 may optionally include atleast one lens 13 to either expand or at least partially collimate thebeam before it strikes the hologram.

The hologram 12 consists of a transparent plastics plate 18 on which areplica of a surface relief interference pattern 14 has been embossedfrom an original master. The pattern itself is protected by a furthertransparent screen 15.

The pattern 14 is a computer-generated interference pattern derived froma mathematical model and conveniently consists of a repeating cell orunit pattern 23 (e.g., see FIG. 5). A detailed report on the productionof such patterns can be found in the aforesaid paper entitled "Efficientoptical elements to generate intensity weighted spot arrays: design andfabrication" (Applied Optics Vol. 30, No 19 pp 2685-2691).

Each cell pattern 22 is designed to produce an array of beams whichtogether form a composite beam having a predetermined shape and/ordistribution in the far field. In one embodiment of the invention theinterference pattern 14 in each cell is a binary phase pattern whichselectively retards the phase of the incident light. By changing thephase of the incident light, the direction of propagation is changed sothat the multiple beams of light emerging from the hologram arepropagated in different scattered directions within the angle of thecomposite beam, making the beam impossible to refocus. For optimumperformance, the hologram 12 should be designed to put as much aspossible of the incident light into the target area with as little aspossible of the light being scattered into higher angles outside thisarea.

The pattern 22 is derived from an algorithm which initially sets therequired far field pattern, compares it with a random pattern of pixels,and assesses the closeness of the fit. Each pixel is then examined inturn to determine whether a change of phase is required to produce acloser fit. The process is repeated many times until a sufficientlyclose fit is achieved.

Since the pattern 22 is a binary phase pattern, each pixel can have onlyone of two phases. With an entirely random phase pattern, the far fieldwould be uniformly illuminated and the hologram would act simply as adiffuser. However, by initially setting the algorithm for the far fieldpattern so that the pattern conforms to a particular shape, such as asquare or circle, the hologram pattern is progressively changed from arandom pattern to a pattern which produces the required shape. At thesame time, the multiple beams of light emerging from the hologram arestill scattered at different angles so that the composite beam cannot berefocused to produce a sharp image of the point source 10.

Three possible examples of a single cell in the interference pattern 14are shown in FIG. 2 together with the resulting light distribution. Ascan be seen, the cell 22a produces a circular far field intensitypattern 30a, the cell 22b produces a square pattern 30b, and the cell22c produces a rectangular pattern 30c. The black areas in each cell 22denote an area with a phase retardance of half a wavelength compared tothe white areas. The angular spread of the beams from each cell is equalto the wavelength of the light divided by the size of the cell.

FIGS. 2a-2c illustrate the far field pattern of light which would beobtained if a single cell 22 were illuminated by a coherent parallellight beam of uniform intensity. The far field pattern 30 represents theintensity of the Fourier transform from one single isolated cell of thephase hologram pattern 14, with about 75% of the light energy falling inthe bright area and the remaining 25% outside. In practice, by repeatingthe cell pattern 22 over more than about 20 cells in a two-dimensionalarray, the Fourier transform becomes an array of spots within therespective bright shaped areas 30a, 30b, 30c shown in FIG. 2 for each ofthe cell patterns 22a, 22b, 22c.

This large two-dimensional array of cells forms the interference pattern14 of FIG. 1. The size of each spot in the far field pattern is equal tothe size of the spot which would appear in the far field without thehologram so that when illuminated by a diverging beam (rather than aparallel beam) the size of the spots is increased although the spacingbetween them is unaffected. If the divergence of the beam issufficiently large (or the spacing between the spots is sufficientlysmall), the spots may merge into one another to form a continuousintensity distribution.

The interference pattern 22 is preferably a phase-only pattern, i.e. itdoes not block any light but just changes the phase. The originalphase-only pattern is produced, for example, by reactive-ion etchingthrough a black/white pattern mask into quartz glass, or by etchingthrough a photo-resist mask directly printed on the glass byelectron-beam lithography. Once the pattern has been recorded on amaster plate, any number of replicas can be made either by embossing thepattern directly on to a low-cost plastics substrate or by firstpreparing a metal negative, for example by plating the quartz glass withsilver and nickel and then peeling off the nickel to form a shim, andsecuring the shim to an embossing roller.

The interference pattern 14 could alternatively be formed as areflection hologram in which case the etch depth would be selected toretard the phase by a quarter wavelength since the light would then passthrough the interference pattern 14 in both directions and the phasewould be shifted twice.

The distance of the target area from the optical source 10 can vary from0.5 meters up to several kilometers.

FIGS. 3-7 illustrate various applications of the device shown in FIG. 1when used as an antenna device, the same reference numerals being usedto denote like parts. Each of these applications is in an optical freespace communication system where the light is carrying atelecommunication signal, such as a television signal.

In FIG. 3 four laser diode sources 10a, 10b, 10c, 10d are used withrespective holograms 12a, 12b, 12c 12d to illuminate four juxtaposedtarget cells 23a, 23b, 23c, 23d.

In FIG. 4 the hologram 12 has an interference pattern 14 designed toproduce a rectangular shaped vertical beam 24 which is used toilluminate a single building 25d in a row of buildings 25a, 25b, 25c an25d.

In FIG. 5 the hologram pattern 14 again produces a rectangular shapedbeam out in this case the beam 28 is horizontal rather than verticalsuch that it illuminates a row of houses in a street.

In FIG. 6 the holographic pattern 14 produces a combination of arectangular shaped beam and a square shaped beam to pick out individualtarget areas and hence illuminate both a group of three houses 26 in astreet as well as a single individual house 27 in the street. The sameprinciple could be used, for example, to illuminate individual officesin a building.

In FIG. 7 multiple laser diode sources 10a, 10b and 10c direct lightthrough a single hologram 12. Each source has a different characteristicwavelength such that the hologram pattern separates the light intoadjacent cells 29a, 29b, 29c corresponding to the different wavelength.In this case the hologram 12 could be a simple diffraction grating sincethe deflection angle of a diffraction grating is proportional to thewavelength. This arrangement is particularly useful where wavelengthdivision multiplexing (WDM) is used for separate data channels. As wellas using such a grating as a common element to separate into adjacentcells the output from a number of sources 10 emitting differentwavelengths, the grating could be used to gather together on to a singledetector the return light at different wavelengths from separate cells,or to separate into different detectors light at several differentwavelengths all emitted from the same cell or spatial location.

Another possibility would be to illuminate the hologram 12 using asingle source 10 and then sweep the frequency of the source (e.g., witha conventional frequency sweep control 30) such that the beam emergingfrom the hologram scans the target area.

The use of a hologram to control the shape of the beam in each of theabove applications has an important additional advantage. Because themultiple beams emerging from each cell of the interference pattern 14are scattered at different angles and therefore pointing in differentdirections, the resulting composite beam from the hologram cannot berefocused into a single point or spot. This means that greatly increasedamounts of power can be transmitted without risk of eye or skin damage.

For example, referring to FIG. 1a, if a converging lens 16 (such as aneye lens) is positioned in the path of the composite beam 20 emergingfrom the hologram 12, a two dimensional array of images of the source 10would be formed in the back focal plane 19 of the lens. The distributionof intensity in these images is determined by the individual cellpattern 22. The images would be separated by a distance (Dλ)/L where λis the wavelength of the source 10, D the working distance of the focalplane from the lens, and L the width of a single cell or unit in therepeating hologram pattern 14.

Because the source 10 generally emits a narrow range of wavelengthsrather than a single wavelength, the images in the back focal plane 19will be smudged. The greater the range of wavelengths, the greater isthe smudging.

To a good approximation, the image in the back focal plane 19 is givenby the Fourier Transform of the phase transmittance of the hologram 12times a phase factor determined by the distance of the lens 16 from thehologram. If the Fourier Transform is such as to produce an extendedimage in the back focal plane 19, the light will appear to the viewer asif it came from an extended object. The extent of the effective objectis given by the extent of the image divided by the magnification of thelens arrangement.

If the hologram 12 produces a uniform N×N square array of beams, theimage in the back focal plane 19 (i.e. the target area covered by thearray) has a size N (Dλ)/L. If P is the incident power collected by thelens, and the working distance D is 10 mm (which is the distance of theretina from the eye lens at close focus), the power density in theretina is given by P×(L² /λ² /N²)×10⁴ W/m². The power density cantherefore be controlled by the hologram pattern--the smaller the unitcell size L, or the greater the number of beams generates, the lowerwill be the power density. It is possible to achieve a reduction of 2500times in the maximum cower density in the retina compared with a systemtransmitting the same total power without a hologram.

The limit to the effectiveness of the hologram 12 in diffusing the lightis the effective bandwidth of the source 10. In addition, a small phaseerror in the hologram gives rise to a small portion of the light beingundeflected by the hologram. This portion can be focused by the lens 16.The phase error can be caused by either an error in the depth of thepattern in the surface, a difference in the source wavelength from thedesign value, or lithographic errors in the processing. Its effect issmall, however;--a 10% error in the profile depth or the sourcewavelength leads to less than 1% of the power being undeflected andwould therefore still allow a 100 fold improvement in the amount ofpower that can be safely emitted.

We claim:
 1. An optical device for emitting a beam of optical radiationto irradiate a remote target area, the device including;a coherentoptical source, a hologram pattern positioned in the path of a lightbeam originating from the source, the source emitting radiationpredominately in a wavelength band having an upper limit less than twicethe lower limit, and the hologram pattern scattering the incidentwavefront into a multiplicity of beams at different angles and out ofphase with one another, the multiple beams together forming anincoherent composite beam which illuminates the remote target area andwhich cannot be refocused by a lens to reproduce an image of the source.2. A device as in claim 1 in which the hologram pattern is a binaryphase pattern.
 3. A device as in claim 1 in which the hologram patternis a surface relief pattern.
 4. A device as in claim 3 in which thesurface relief pattern is a replica derived from an original masterpattern.
 5. A device as in claim 4 in which the original master patternis prepared on a first substrate and the replica is impressed on asecond substrate.
 6. A device as in claim 5 in which the hologrampattern is formed over the area of the second substrate by repeatedlyimpressing the master pattern over successive portions of the area.
 7. Adevice as in claim 5 in which the hologram pattern is formed bycombining a plurality of replica patterns derived from one originalmaster.
 8. A device as in claim 5 in which the second substrate is atransparent plastics substrate.
 9. A device as in claim 1 in which thehologram pattern is a repeating pattern of a single cell.
 10. An opticalfree space communication system comprising a device as in claim 1wherein the radiation incident on the hologram pattern is modulated witha telecommunication signal and the target area is spaced at least 0.5meters from the optical source.
 11. A system as in claim 10 furtherincluding a plurality of other optical sources having differentwavelength bands, and wherein:the hologram pattern receives light fromeach of the sources and separates the incident light into respectivecomposite beams corresponding to the different wavelength bands and eachilluminating a respective target area.
 12. A system as in claim 10further comprising;means for sweeping the frequency of the sourcethrough a continuous range of frequencies whereby the composite beamemerging from the hologram pattern scans the target area.
 13. A systemas in claim 10 in which the optical source is located outside at leastone building and the target area comprises at least a portion of thesaid building or buildings.
 14. A system as in claim 10 in which theoptical source is located within a room or chamber and the target areais a predetermined sector of the room or chamber.
 15. A system as inclaim 10 in which the target area is at least 0.5 m².
 16. A free-spaceoptical signal transmission system having reduced safety hazards forliving organisms located in a free-space segment of an optical signalpath, said system comprising:a coherent optical signal source providinga first optical beam; an optical beam expander disposed to expand thecross-sectional area of said first optical beam; and a plurality ofholographic patterns positioned in the path of the expanded area firstoptical beam to produce a plurality of dispersed output optical beams asa composite output beam of expanded cross-sectional area fortransmission through free space in a form which cannot be substantiallyrefocused by a living eye lens to form an image of the coherent opticalsource.
 17. A free-space optical signal transmission system as in claim16 wherein at least some of said holographic patterns havephase-altering patterns to prevent any possibility of the compositeoutput beam being substantially refocused by a living eye lens to forman image of the coherent optical source.
 18. A method for transmittingoptical signals through free space while reducing safety hazards forliving organisms located in a free-space segment of an optical signalpath, said method comprising:providing a coherent first optical beam;expanding the cross-sectional area of said first optical beam; andpositioning a plurality of holographic patterns in the path of theexpanded area first optical beam to produce a plurality of dispersedoutput optical beams as a composite output beam of expandedcross-sectional area for transmission through free space in a form whichcannot be substantially refocused by a living eye lens to form an imageof the coherent optical source.
 19. A method as in claim 18 wherein atleast some of said holographic patterns have phase-altering patterns toprevent any possibility of the composite output beam being substantiallyrefocused by a living eye lens to form an image of the coherent opticalsource.